In one of our recent project. There are multiple generators and individually feeding to each MDBs. Consultant instructed to loop all the Generator Neutral between each generators. Our proposal to provide individual neutral earthing for each generators.
Is it advisable to loop Generator Neutral Earthing (These generators are not synchronizing)
I'm not sure I've pictured this correctly, but...
It sounds like you have several generators, but they're feeding completely separate systems - their outputs aren't being paralleled. In which case I'd expect there to be a N-PE link for each generator (if they were paralleled, there should be one link for all the generators connected together at any one time). If the overall systems (generators and everything they supply) are anywhere within reach of each other (e.g. within the same building) then I'd expect them to share the same earthing/bonding arrangements - which would in effect connect all the neutrals together. (If there were separate systems out of reach of each other you could have separate electrodes for each generator if you wanted, or still combine them altogether, whichever was least inconvenient).
Perhaps another option in theory is to have a combined N & PE conductor shared between the systems - but as that's prohibited on consumer's installations in the UK (by law), it's not a common way to do things here (they legal entity running the generators would have to be different to the consumer), but might be more usual elsewhere.
- Andy.
If you do this wrong you can end up firing the earth fault relay on one genset with the neutral current of another. Also consider carefully how you would ensure one genset was safe to work on for repairs, if the others are still running, and the neutral is common.
For these reasons normally I'd expect one N-E link per genset, and a common or at least cross- linked earth arrangement BUT NOT a COMMON NEUTRAL except between pairs of gensets in tandem with no Earth fault protection for the tandem wiring.
Gensets synced or not makes no difference, though unusual and it is worth avoiding phases from different gensets coming to the same switch gear, as the voltage between them is indeterminate.
I think you need to query the reason with the consultant, and how he or she expects the earth trips to be set, and maintenance on any one genset to be performed if there is any shared neutral wiring.
Mike.
PS
And that bit of the ESQCR
"(4) A consumer shall not combine the neutral and protective functions in a single conductor in his consumer’s installation. "
Does not preclude multiple NE links in the generation side, but not in the load side.
You have probably made an assumption of these being "packaged" generators Mike. In a fixed installation Earth fault relays would not be fitted at these machines, but in or after the MDB or maybe not at all. However you are right about the separation for maintenance etc, and there is no reason I can think of as to why the neutrals should be connected before the MDB and therefore the maintenance isolation.Looking at the photo, I think this is likely not to be UK, and here unsynchronised generators would be very unusual, but the ESCQR would not apply. Again CNE etc and cables to the MDB would probably be short so not worthwhile. Likewise the position of N-E links may well be the MDB.
I cannot see a photo. So yes, I was imagining each genset with its own regulation, main breakers etc. I'm also assuming generation direct at LV for distribution - So if it is multi-megawatt with generation at 6000v per phase I'm out of my depth, but I'd still expect there to be some sort of earth fault detection, after all we manage it on our 11kv overhead lines.
Mike.
The answer to all this is that we don't know Mike. Usually there is an assumption that protection of the generator output cables themselves is not required (it is not there in packages either) because winding faults are much more likely and these cables can be safely looked after (sheathed and in trunking for example). Generator shorts to earth are power limited by the engine anyway, and fusing which blows on short at this point is difficult to arrange (10X current is simply not available for any useful period), and the current may be very large at 230/400V say 1500A per phase a 1MW set. The current may also be monitored by the excitation circuits via a CT, and then electronic load limiting may be arranged.
The MCCB or RCD for small sets on the ones you have seen is there for load protection just like any installation, We need a low enough Ze value to operate CPDs on an Earth fault outside the generator, it cannot protect the innards. As a large excess current is not available we use other methods of Earth fault detection, but this is not Additional Protection at 30 mA, except possibly on the smallest generators, say 60 kVA, because there will be significant Earth current if the installation is of any size, we may use 30mA on the final circuits, but leakage on a 1MW set driving a hospital could be several amps, because there are hundreds of appliances. The distribution circuits are protected against faults by being TN-S, with suitable CPDs to suit the expected current.
When generators are operated in parallel there may be significant currents between them caused by slight differences in engine phase and exact load, and throttle response times. However this does not appear to be relevant here.
Hi Mr.AJJewsbury
In which case I'd expect there to be a N-PE link for each generator - There is no N-PE link, Neutral and Body Earthing are separate network. Our projects in UAE and its a Data Center - Tier 3 Project.
there should be one link for all the generators connected together at any one time).- Each MDBs are connected separately and a separate network path to final Equipments/ Rack to maintain redundancy.
If the overall systems (generators and everything they supply) are anywhere within reach of each other (e.g. within the same building) -Yes 4 MDBs are fed by 4 individual Generators as well as Transformers.
(If there were separate systems out of reach of each other you could have separate electrodes for each generator ) - Separate Neutral earthing was considered for individual Generators and Body earthing for individual generators are lopped in between generators.
Our projects in UAE
So what standard(s) apply?
BS 7430: Code of Practice for Earthing
BS 7671: Requirements for Electrical Installations
NFPA 70: National Electrical Code.
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